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GCEP researchers at Stanford University have improved a polymer sorting technique to separate semiconducting carbon nanotubes from metallic ones. The technique also produces a larger number of smaller-diameter semiconducting nanotubes. These smaller tubes have larger electronic bandgaps and are thus ideal for solar cell applications.
A single-walled carbon nanotube (SWNT) is a sheet of carbon just one atom thick that has been rolled up into a tube with a diameter of about 1 nanometer. The atoms in the sheet are arranged in a hexagonal lattice. The relative orientation of the lattice to the axis of the tube – or its "chirality" – dictates whether the tube is a metal or a semiconductor. Semiconducting tubes can be used to build transistors, for example, and some scientists even believe that these structures could replace silicon in future electronic devices, because they are tiny but can still carry huge amounts of current.
Although SWNTs are relatively easy to grow, sorting them according to whether they are metallic or semiconducting is difficult, costly and time consuming. Now, a team led by GCEP investigator Zhenan Bao, Stanford professor of chemical engineering, has developed a way of isolating different types of nanotubes by mixing them with regioregular poly(3-alkylthiophenes) polymers. According to the researchers, the sorting process is quite simple and involves putting the polymer and SWNT mixture in an ultrasonic bath, followed by centrifuging. This process is highlighted in a journal ACS Nano.
"The major improvement compared to previous such work is that we used the polythiophene with longer side-chains to produce a higher sorting yield of small-diameter semiconducting nanotubes," team member Huiliang Wang, a graduate student at Stanford. "Molecular dynamics simulations performed by our colleagues Peng Liu and Gonzalo Jimenez-Oses at UCLA showed that the higher yield comes thanks to an increased surface interaction area between the longer side-chain polymers and the SWNTs."
Larger bandgaps ideal for efficient electron and hole separation
In addition to segregating semiconducting tubes, the technique also isolates more smaller-diameter SWNTs. These tubes have larger bandgaps – which is ideal for efficient electron and hole separation in solar cells, especially in single-junction devices, said team member Ghada Koleilat, a postdoctoral fellow at Stanford.
The researchers looked at CoMoCAT (CO disproportionation on Co–Mo catalysts) SWNTs in their study. CoMoCAT tubes have a high open-circuit voltage and infrared external quantum efficiency, which measures the ratio of generated charge carriers (electrons and holes) to the incident photons falling on the structures. Indeed, the open circuit voltage of 0.44 V in the CoMoCAT tubes is the highest ever reported for solar cells that exploit SWNTs as light absorbers.
The GCEP team now is busy trying to better understand how the structure of polymer affects SWNT sorting. Their goal is to find the best type of polymer for separating nanotubes. "We hope to do this by designing new polymers and with detailed molecular dynamics simulations," Wang said.
Feb 26, 2014
Adapted from an article that was originally published in nanotechweb.org and written by Belle Dumé, their contributing editor.
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